When universal motors are controlled with the aid of a phase control system, current and voltage are not necessarily in phase (cos .noteq.1). If a triac were to be supplied with a trigger pulse during a lag phase and thus at a point in time at which the triac is still carrying current, this would lead to gaps in the load current. To avoid this undesirable effect, a current detector is used in phase control systems to determine whether current flows through the triac or not.
In the past, current flow in a triac has been detected by using the voltage across the triac employed as a power switch, i.e., the voltage between terminals H1 and H2. However, this known current flow detection has the drawback that a separate circuit input and an external resistor are needed to detect the voltage across the triac. Since present-day phase control systems are known to be integrated, a separate circuit input and an external resistor involve additional expenditures which are not desirable.